The present invention relates to high temperature gas seals, particularly for use in a planar solid oxide fuel cell stack.
A planar solid oxide fuel cell (pSOFC) stack has three primary constituents: a ceramic electrochemical cell membrane, metallic interconnects, and an arrangement of seals. To perform the function of converting chemical energy into electrical energy, a SOFC membrane must have one electrochemical face exposed to an oxidant gas, and the other exposed to a fuel gas, all at an operating temperature above 500° C. A metallic interconnect (IC) provides fuel and oxidant gas distribution to the cells by means of separate plenums, and when arranged between cells in a stack arrangement, also transfers electrical current from one cell to another. The seals required between a ceramic cell and an interconnect in a SOFC stack must provide adequate resistance to gas permeation to contain the reactants within the gas distribution plenum, as well as provide adequate electrical isolation. The seal should preferably resist significant degradation over time, and should preferably be capable of being thermally cycled.
There are essentially two standard methods of sealing: (1) by forming a rigid joint or (2) by constructing a compressive “sliding” seal. Each method has its own set of advantages and design constraints.
Rigid joints utilizing glass joining is a simple method of bonding ceramic to metal. However, the softening point of glass limits the maximum operating temperature. In addition, because the glass-ceramic is a brittle material and forms a non-dynamic, low-yielding seal, it is imperative that the temperature dependent coefficient of thermal expansion (CTE) for each of the joining components, i.e. the ceramic cell, the seal, and the metallic IC, be approximately equal. If not, high thermal stresses can develop within the components during stack heat-up and/or cool-down, causing fracture of the cell or seal. Only a narrow range of high temperature glass compositions within the borate- or phosphate-doped aluminosilicate families display coefficients of thermal expansion that match those of the ferritic stainless steels commonly employed in stack interconnects and housings. Unfortunately, these glasses typically display signs of devitrification within the first few hours of exposure at operating temperature. As the glass begins to crystallize, its carefully engineered thermal expansion properties change significantly, ultimately limiting the number of thermal cycles and the rate of cycling at which the resulting joint is capable of surviving. Even if the coefficients of thermal expansion are matched, non-uniform thermal expansion can still result, as the thermal conductivities of the stack components are typically not matched. As glass is an inherently brittle material, it cracks and fails under thermal cycling conditions, and as a result of jarring shocks or vibrations, which is often the case in mobile applications.
A further disadvantage of glass seals is that they can have a chemical incompatibility with electrocatalytic cells, leading to performance degradation during operation. SOFCs are particularly sensitive to alkali elements contained in many glass seals, which have been found to detrimentally affect the SOFC catalyst. Glass composition and phase shifting due to interaction with contact materials is also a problem for long-term service.
Compressive sealing is an alternative method. A compliant high-temperature material is captured between the two sealing surfaces and compressed, using a load frame external to the stack, to deliver sealing in the same way rubber gaskets are used in everyday appliances. Because the seal conforms to both sealing surfaces and is under constant compression during use, it forms a dynamic seal. That is, the sealing surfaces can slide past one another without disrupting the seal property and CTE matching is not required between the ceramic cell and the metallic IC. However, compliant seals of this nature suffer from the disadvantage of inadequate sealing performance, primarily due to the lack of a reliable high-temperature sealing material that would form the basis of the compliant seal. A number of materials have been considered, including mica, nickel, and copper, but each has been found deficient for any number of reasons, ranging from oxidation resistance in the case of the metals to poor hermeticity and through-seal leakage with respect to the mica. In the case of mica, while being able to withstand high temperatures, the natural variance in thickness of mica sheets and the relative non-compressibility of the mica both contribute to this poor sealing behavior. Additionally, it has been found that the mica may leach minerals that can poison the catalyst in the cell.
Therefore, there is a need in the art for a seal suitable for use in a high temperature fuel cell that mitigates the difficulties found in the prior art.